A group-III nitride semiconductor has a direct transition-type band gap of an energy equivalent to a range of an ultraviolet light region from visible light, is commercialized as a semiconductor light-emitting devices such as a light-emitting diode (LED) or a laser diode (LD) because of an excellent emission efficiency, and is used for various applications. In addition, even when used in electronic devices, the group-III nitride semiconductor has the potential of obtaining more excellent properties than when the groups-III-V compound semiconductor of the related art is used.
Such a group-III nitride semiconductor is generally manufactured by a molecular beam epitaxy (MBE) method, in addition to those manufactured by a metalorganic chemical vapor deposition (MOCVD) method, using trimethyl gallium, trimethyl aluminum and ammonia as raw materials. The MOCVD method is a method of growing a crystal by containing vapor of the raw materials in a carrier gas to carry the vapor to the surface of a substrate, and decomposing the raw materials at the heated surface of the substrate.
In the general light-emitting devices in which a group-III nitride semiconductor is used, an n-type semiconductor layer, an emission layer and a p-type semiconductor layer, which are made of a group-III nitride semiconductor, are laminated on a sapphire single-crystal substrate in this order. Since the sapphire substrate is an insulator, the structure of the device generally has a structure in which a positive electrode formed on a p-type semiconductor layer and a negative electrode formed on an n-type semiconductor layer exist on the same surface. In such a group-III nitride semiconductor light-emitting device, there are two types of devices of a face-up type of extracting light from the p-type semiconductor side by using a transparent electrode in the positive electrode, and a flip-chip type of extracting light from the sapphire substrate side using a high reflective film made of Ag and the like in the positive electrode.
As a barometer of the output of the light-emitting device mentioned above, the external quantum efficiency is generally used. If the external quantum efficiency is high, it can be said that the light-emitting device has a high emission output. In addition, the term “external quantum efficiency” means a barometer of multiplication of the internal quantum efficiency by the light extraction efficiency, and the term “internal quantum efficiency” means the ratio of conversion of energy of a current implanted in the device into light at the emission layer. In addition, the term “light extraction efficiency” means the ratio of light capable of being extracted to the outside of the light-emitting device in light which is generated at the emission layer. Therefore, it is necessary to improve the light extraction efficiency in order to enhance the external quantum efficiency.
As methods of improving the light extraction efficiency, there are mainly two methods. One is a method of reducing the absorption of light by an electrode and the like formed on the light extraction surface, and the other is a method of reducing the confinement of light into the inside of the light-emitting device generated due to a difference of refractive indexes between the light-emitting device and the external medium thereof.
In addition, as a method of reducing the confinement of light into the inside of the light-emitting device, a technique for forming the asperity on the light extraction surface of the light-emitting device is proposed (see, for example, Patent Document 1).
However, in the light-emitting device in which the asperity is formed on the light extraction surface by mechanical processing or chemical processing as disclosed in Patent Document 1, there may be a concern that the load is applied to the semiconductor layer by performing processing on the light extraction surface, and thus damage remains in the emission layer. In addition, in the light-emitting device in which the semiconductor layer is grown in the manufacturing conditions of forming the asperity on the light extraction surface, crystallization of the semiconductor layer is deteriorated, and thus defects are included in the emission layer. For this reason, when the asperity is formed on the light extraction surface, there has been a problem that while the light extraction efficiency is improved, the internal quantum efficiency is lowered, and thus the emission intensity cannot be enhanced.
Consequently, instead of a method of forming the asperity on the light extraction surface, there is proposed a method of forming the asperity on the surface of a sapphire substrate, and growing a group-III nitride semiconductor layer thereon (see, for example, Patent Document 2). According to this method, the asperity is formed at the interface between the sapphire substrate and the group-III nitride semiconductor layer, and thus it is possible to reducing the confinement of light into the inside of the light-emitting device due to the diffused reflection of light generated at the interface by a difference of refractive indexes between the sapphire substrate and the group-III nitride semiconductor layer, and to improve the light extraction efficiency.
However, in the method disclosed in Patent Document 2, there has been a problem that while the light extraction efficiency from the semiconductor layer on the sapphire substrate can be improved, the light extraction efficiency from the sapphire substrate cannot be improved.
Further, there is known the light-emitting device in which the lateral side of the group-III nitride semiconductor layer is inclined with respect to a normal line of the main surface of the substrate, in order to provide the light-emitting device having a high light extraction efficiency (see, for example, Patent Document 3). Patent Document 3 discloses the use of a laser as means for removing the nitride semiconductor layer until it reaches the substrate.
On the other hand, as a method of dividing a wafer into individual devices, there is proposed a method of forming a reforming region through laser irradiation focusing a light collecting point on the inside of the substrate of the wafer on which the semiconductor layer is laminated, forming a cutting start point region through this reforming region, and cutting the wafer along the cutting start point region (see, for example, Patent Documents 4 and 5).
[Patent Document 1] Japanese Patent No. 2836687
[Patent Document 2] Japanese Unexamined Patent Application Publication No. 2002-280611
[Patent Document 3] Japanese Unexamined Patent Application Publication No. 2006-253670
[Patent Document 4] Japanese Unexamined Patent Application Publication No. 2003-338468
[Patent Document 5] Japanese Unexamined Patent Application Publication No. 2006-245062